Yes "Walk" and "Walk Fast" are designed to change anim automatically when move speed changes.
In this model, walk speed is set to 100 and run speed is set to 240.
So, what you should do in unit editor is setting variables "Art-anim-walk speed" (uwal) = 100 and "Art-anim-run speed" (urun) = 240.
After that, if unit move speed is close to 240 it should use "walk fast" anim.
And if the unit gets slowed by a spell for example and its actual move speed gets close to 100, it will switch to "walk" anim.
I didn't check at which move speed value the change actually happens, but it's probably at the average between the 2 values (in that case, 170).